Multi-channel time-division multiplexing access methods and systems

ABSTRACT

Devices, systems and methods for high-utilization low-latency multi-channel time-division multiplexing access (TDMA) are described. One example method for wireless communication includes performing, in a first time interval of a time-division multiple access (TDMA) slot, a transmission of a first data unit over a first logical channel of the plurality of logical channels, refraining from transmitting, subsequent to a completion of the transmission of the first data unit, for a second time interval immediately after the first time interval, and performing (N−1) transmissions in (N−1) time intervals for each data unit of (N−1) subsequent data units in the TDMA slot, such that a transmission of an nth data unit of the (N−1) subsequent data units is performed over an nth logical channel of the plurality of logical channels.

TECHNICAL FIELD

This document is directed to collaborative wireless communicationsamongst nodes in a wireless network.

BACKGROUND

In certain wireless communication applications, signals travelingbetween nodes can encounter long propagation delay. For an example,using geosynchronous satellites to communicate between mobile stationsand base stations across a continent may result in hundreds ofmilliseconds of delay. In another example, the propagation delay of anunderwater acoustic channel across the depth of an ocean can be manyseconds. Traditional time-slotted medium access control (MAC) schemestrade-off channel utilization to accommodate these large propagationdelays. Enabling high-utilization and low-latency communication in thesechannels can significantly expand emerging communication capabilities.

SUMMARY

This document relates to high-utilization low-latency multi-channeltime-division multiplexing access (TDMA). Embodiments of the disclosedtechnology can be configured to provision multiple bursts over multipleorthogonal radio resources during a single TDMA slot, whichadvantageously increases the link utilization and reduces timinguncertainties.

In an exemplary aspect, a method for wireless communication over awireless medium comprising a plurality of logical channels is disclosed.The method includes performing, in a first time interval of atime-division multiple access (TDMA) slot, a transmission of a firstdata unit over a first logical channel of the plurality of logicalchannels, refraining from transmitting, subsequent to a completion ofthe transmission of the first data unit, for a second time intervalimmediately after the first time interval, and performing (N−1)transmissions in (N−1) time intervals for each data unit of (N−1)subsequent data units in the TDMA slot, such that a transmission of annth data unit of the (N−1) subsequent data units is performed over annth logical channel of the plurality of logical channels, wherein n andN are positive integers and 2≤n≤N, wherein each of the plurality oflogical channels corresponds to a distinct transmission resource,wherein the wireless medium is characterized by a maximum propagationdelay, and wherein a duration of the TDMA slot is greater than themaximum propagation delay.

In another exemplary aspect, a method for wireless communication over awireless medium comprising a plurality of logical channels is disclosed.The method includes receiving and detecting, at a first time in atime-division multiple access (TDMA) slot, a first data unit over afirst logical channel of the plurality of logical channels, andreceiving and detecting at least one of (N−1) subsequent data units ateach of (N−1) times in the TDMA slot, such that a reception of an nthdata unit of the (N−1) subsequent data units is performed over an nthlogical channel of the plurality of logical channels, wherein n and Nare positive integers and 2≤n≤N, wherein each of the plurality oflogical channels corresponds to a distinct transmission resource,wherein the wireless medium is characterized by a maximum propagationdelay, and wherein a duration of the TDMA slot is greater than themaximum propagation delay.

In yet another exemplary aspect, a wireless communication system isdisclosed. The system includes a transmitter, a first receiver coupledto the transmitter over a wireless medium comprising a plurality oflogical channels, wherein each of the plurality of logical channelscorresponds to a distinct transmission resource, wherein the wirelessmedium is characterized by a maximum propagation delay, and wherein adistance between the transmitter and the first receiver corresponds to afirst propagation delay that is less than the maximum propagation delay,and a second receiver coupled to the transmitter over the wirelessmedium, wherein a distance between the transmitter and the secondreceiver corresponds to a second propagation delay that is less than thefirst propagation delay, wherein the transmitter is configured toperform, in a first time interval of a time-division multiple access(TDMA) slot, a transmission of a first data unit over a first logicalchannel of the plurality of logical channels to the first receiver andthe second receiver, refrain from transmitting, subsequent to acompletion of the transmission of the first data unit, for a second timeinterval immediately after the first time interval, and perform (N−1)transmissions in (N−1) time intervals for each data unit of (N−1)subsequent data units in the TDMA slot, such that a transmission of annth data unit of the (N−1) subsequent data units is performed over annth logical channel of the plurality of logical channels, wherein thesecond receiver is configured to receive and decode, within the TDMAslot, at least one of the N subsequent data units, wherein the firstreceiver is unable to receive and decode each of the N subsequent dataunits, wherein a duration of the TDMA slot is greater than the firstpropagation delay, and wherein n and N are positive integers and 2≤n≤N.

In yet another exemplary aspect, the above-described methods areembodied in the form of processor-executable code and stored in acomputer-readable program medium.

In yet another exemplary embodiment, a device that is configured oroperable to perform the above-described methods is disclosed.

The above and other aspects and their implementations are described ingreater detail in the drawings, the descriptions, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C illustrative slot assignments for multi-hopnetworks.

FIG. 2 shows an example of a broadcast flooding protocol for barragerelay networks.

FIGS. 3A, 3B and 3C show examples of TDMA slot utilizations.

FIG. 4 shows an example of a transmitter and multiple receivers usingmultiple channels in a single TDMA slot.

FIG. 5 shows an example of multiple cooperative transmitterssimultaneously using different channels the same TDMA slot.

FIG. 6 shows a flowchart of an example method for wirelesscommunication.

FIG. 7 shows a flowchart of another example method for wirelesscommunication.

FIG. 8 is a block diagram representation of a portion of a radio thatmay be used to implement embodiments of the disclosed technology.

DETAILED DESCRIPTION

Propagation delay is the amount of time it takes a communication signalto travel from the source to the destination over a given transmissionmedium, i.e., D_(p)=d/c, where D_(p) is the propagation delay, d is thedistance between the source and the destination, and c is the speed ofthe signal. In most terrestrial wireless systems, such as mobilecellular and WiFi networks, the propagation delay is typically smallcompared to the packet size, and the effects of propagation delay can bemitigated using techniques such as guard periods. However, using a guardtime in systems with long propagation delays (e.g., underwater acousticchannels, satellite communications, etc.) results in poor channelutilization. Furthermore, configuring a system to use the worst-caseround-trip time (RTT) produces significant inefficiencies for devices inthe system that have RTTs that are smaller than the worst-case RTT.Embodiments of the disclosed technology enable high-utilization andlow-latency communication in time-slotted networks over channels withlong propagation delays.

Some embodiments described herein are directed towards single-hop andmulti-hop, time-slotted wireless networks. That is, a wireless networkthat may implement a time-division multiple access (TDMA) scheme thatdivides a unit of time, e.g., one second, into slots, each of which arededicated for the transmissions and reception of messages from nodesthat may be multiple hops from each other.

In an example, the representative slot assignments shown in FIGS. 1A, 1Band 1C define virtual channels for different types of messages includingsynchronization, data and voice messages. Table 1 provides a legend forsome of the types of slots assigned within a frame.

TABLE 1 Logical channels used in slot assignments S Synchronizationlogical channel C Clear-to-send logical channel R Request-to-sendlogical channel N Network maintenance logical channel D Data logicalchannel V Voice logical channel

A Barrage Relay Network (BRN), which is an example of a time-slotted,multi-hop wireless network, is shown in FIG. 2 . The BRN in FIG. 2illustrates a wireless network where independent medium allocations areobtained via a TDMA scheme. While BRNs can be defined according tovarious medium allocation schemes (e.g., time-slotting, differentfrequency channels, different frequency-hopping patterns, differentantenna radiation patterns, low cross-correlation spreading sequences,and the like), embodiments of the disclosed technology are described inthe context of a time-slotted barrage relay network but are intended tobe applicable to other medium allocation schemes.

In particular, time is divided into frames, which are further dividedinto multiple slots per frame (for example, FIG. 2 employs 3 slots perframe labeled “A,” “B” and “C”). The data that is transmitted in a giventime slot is denoted a “packet.” Two packets that are transmitted by twodifferent nodes are said to be identical if all data—including allprotocol header information—contained in the respective packets isidentical.

In an embodiment, for example, a central node 101 transmits a packet onslot A of the first TDMA frame. All nodes that successfully receive thispacket are, by definition, one hop away from the source. These nodes arelabeled 111-117 in FIG. 2 . These nodes transmit the same packet on slotB, thus relaying to nodes that are two hops away from the source (nodes121-129), which in turn transmit the same packet on slot C. Nodes thatare three hops away from the source node (nodes 131-137) relay thepacket on slot A of the second TDMA frame. Thus, packets transmitoutward from the source via a decode-and-forward approach.

In the embodiment shown in FIG. 2 , a number of two-hop nodes receivethe same packet from different one-hop nodes. These packets do notcollide due to the physical (PHY) layer processing employed by BRNs. Inparticular, BRNs employ a PHY layer that allows identical packets to becombined at the receiver in a manner analogous to multipath mitigationin traditional radio receivers. That is, the multiple, time-shiftedcopies of the received signal that arise in BRNs can be interpreted atthe receiver as resulting not from different transmitting nodes, butfrom reflections off, for example, buildings when a single sourcetransmits.

In order for two packets to be identical, both the payload data and allprotocol header data must be identical. Therefore, protocol headers in abarrage relay network can be modified only in a manner that is commonacross all nodes at a given hop distance from the source. This is instark contrast to traditional layered network architectures that employa point-to-point link abstraction at Layer 2, wherein protocol headerscan be modified in a node-specific—as opposed to a hop-specific—manner.

In some embodiments, the spatial reuse of time slots enables packets tobe pipelined into the source for transmission every three slots.Specifically, as shown in FIG. 2 , the one-hop nodes will not receivethe packet broadcast by the three-hop nodes during slot A of the secondTDMA frame. Thus, the source can safely transmit a second packet duringthat slot. In this manner, a throughput of W/3 can be achieved forbroadcast in a single-source BRN (wherein W is the capacity of a singlepoint-to-point link). This efficient injection of messages for broadcasttransmission is denoted “spatial pipelining” in order to highlight itsreuse of time slots between spatially separated nodes.

More generally, spatial pipelining can be achieved by having a sourcenode inject a new packet for every barrage relay broadcast every M slotsresulting in a throughput of W/M. In this context, M is referred to asthe spatial pipelining factor (SPF). In some embodiments, when the sizeof an arbitrary wireless network is not known to the source a priori, Mmust be at least 3 to avoid collisions. Larger spatial pipeliningfactors (e.g., 4) may be chosen in order to enhance robustness in highlymobile network topologies.

Furthermore, in order to contain the extent of a given barrage relaytransmission, two fields can be incorporated into the header (preamble)of each data packet: a time-to-live (TTL) field and a hop count (HC)field. The TTL field is unchanged by relaying nodes while the HC fieldis initially set to 1 by the source of the packet and incremented uponrelay. In the context of FIG. 2 , the central node 101 may set the TTLfield to 8, and enable the packet to propagate over 8 hops through theBRN. The one-hop neighbors of this central node would receive suchpackets and relay a modified packet with the HC field set to 2.Similarly, two-hop neighbors set the HC to 3, and so on. Relayingcontinues whenever a received packet has an HC field that is less thanor equal to the TTL field, but stops if this condition cannot besatisfied.

Although the description of the interaction between the TTL and HCfields is in the context of BRNs, the notion of increasing the HC fieldupon relaying and stopping the relaying process when a packet with equalTTL and HC fields is received is not limited to BRNs, and is in generalcompatible with other wireless networks. For example, a time-slottedmulti-hop network that comprises a single node at each hop can supportthe interaction between the TTL and HC, as well as spatial pipelining,in the manner described above.

In time-slotted networks (e.g., a BRN described above), TDMA-based MACsmay be configured to employ a slot guard time to overcome propagationdelay and timing reference-error to avoid overlap of two consecutivesignals between two consecutive TDMA slots at a receiver. Typically, thepropagation time is the dominant source of timing uncertainty, and theslot guard time is provisioned based on the maximum propagation delay ofthe link.

In terrestrial RF communications, the link propagation delays (100 μsecfor 30 km) are typically small compared to signal transmission (burst)time (hundreds of microseconds to milliseconds), therefore the relativeoverhead due to slot guard-time is manageable. However, there areapplications for which long propagation delays are unavoidable. Forexample, line-of-sight airborne networks may operate over link distancesof many hundreds of kilometers, for which milliseconds of guard time maybe necessary. Underwater acoustic communications (UWAC) is also subjectto large propagation delays due to relatively slow speed of propagationof waves (˜1.5 km/sec), translating to seconds of delay (e.g., to covermultiple kms spanning the ocean depth).

FIGS. 3A, 3B and 3C illustrate examples of TDMA slot allocations. Asshown in FIG. 3A, the TDMA slot (310) has signal duration S (320),maximum propagation delay P (330), and slot guard time G (340), whereG>P. In this example framework, the slot-level channel utilization, U,is determined as:

$U = {\frac{S}{T} = {\frac{S}{S + G} \sim {1 - {\frac{P}{S + P}.}}}}$

The slot-level latency, L, is determined as:

${{L \sim T} = {{S + G} \sim {S\left( {1 + \frac{P}{S}} \right)}}}.$

In some embodiments, the MAC layer system utilization and latency areproportional to the slot-level system utilization and latency metrics,respectively. In general, a smaller S and a smaller P S ratio aredesirable for a TDMA MAC.

FIG. 3A illustrates an example TDMA scheme with a small P (compared tothe signal duration, S) that operates with high efficiency and lowlatency. FIG. 3B illustrates another example TDMA scheme with a large P(compared to the TDMA slot length, T) and operates with comparablelatency but lower utilization. FIG. 3C illustrates yet another TDMAscheme with a large P with comparable utilization but high latency. Thetraditional TDMA schemes are therefore subject to a trade-off betweenutilization and latency for large P.

In some embodiments, and for scenarios with large propagation delay,random access MAC solutions may be implemented to improve the channelutilization, but this comes at the expense of the desirable attributesof TDMA.

The present document uses terminology that includes physical layerprotocol data units (PPDUs) and interframe spacing (IFS) only tofacilitate understanding and the disclosed techniques, and embodimentsmay be practiced in other wireless systems that use physical layerbursts and inter-burst spacing.

Embodiments of the disclosed technology are directed to, for example,communication systems subject to high propagation delay, and may beconfigured to achieve high channel utilization and low latency (down tothe signal duration, S) by provisioning multiple physical layer protocoldata units (PPDUs) (or more generally, multiple bursts) over multipleorthogonal radio resources during the TDMA slot. The radio resourceshere are referred to as channels or logical channels. In an example, thelogical channel may be configured in frequency, in space (antennadirectivity), in polarization, or in code domains.

According to the described embodiments, high utilization and low latencycan be achieved irrespective of how slots are organized and utilized atthe TDMA frame level, and the described methods and techniques can beapplied to existing radios with a single half-duplex transceiver (thatenables the radio to either transmit or receive, but not both at thesame time).

In some embodiments, the sender of a TDMA slot transmits the first PPDUusing a primary channel. Instead of idling for the remaining channeltime within the transmitting slot, the sender switches to one or moresecondary channels to transmit additional PPDUs, each separated in timeby a predefined interframe spacing (IFS), long enough to accommodate thetime for transmitter and/or receivers to switch from one channel toanother. All transmitted PPDUs are fully contained within the currentTDMA slot. The larger the maximum propagation delay (P), the larger theTDMA slot, and therefore the more PPDUs may be transmitted within thesingle TDMA slot. From the perspective of this transmitter, thepreviously unutilized guard time is fully utilized for dissemination ofinformation in the additional PPDUs.

In some embodiments, one or more specific logical channels in the BRNcan be used to transmit multiple PPDUs as described in the embodimentsherein, and other logical channels could be used as originally designed.In an example, multiple PPDUs may be transmitted in the framesassociated with only a synchronization logical channel (e.g., “S” inFIG. 1A), and the other channels can be used for data (e.g., “D” in FIG.1A), voice (e.g., “V” in FIG. 1A), etc.

In some embodiments, the receivers of a TDMA slot always tune to theprimary channel and look for the first PPDU at the start of the TDMAtime slot. Upon receiving the first PPDU, each receiver evaluates thetime-of-arrival (TOA) and independently decides whether it shall switchto the secondary channel(s) for receiving the subsequent PPDU(s). If theTOA of the primary PPDU is too late, (due to the time offset withrespect to the sender and the propagation delay), a specific receivermay decide to remain on the primary channel and prepare for the upcomingactivity in the next time slot. Similarly, each receiver mayindependently decide to receive all or a subset of the subsequentPPDU(s) based on its local timeline.

FIG. 4 illustrates an example embodiment of the described technologyconfigured for a transmitter (TX) and multiple receivers (RXn). Thetransmitter sends a TDMA slot of length T (410) by transmitting a firstPPDU (412) in the primary channel (CH1), followed by one or more PPDUs(414, 416, 418) on alternate channels (CH2, CH3 and CH4, respectively).The nth PPDU, denoted by Sn, is followed by an interframe spacing IFSn(413, 415, 417 and 419 for n=1, 2, 3 and 4, respectively). In someembodiments, IFSn is selected to be large enough to accommodate the timerequired to switch between channels, the maximum delay spread on CHn, aswell as the receiver processing latency of a PPDU. In some embodiments,there is no explicit slot-guard governed by the propagation times; infact, IFSn can be much smaller than the maximum propagation delay onCHn, e.g., tens of milliseconds vs seconds for the underwater acousticchannels. If the propagation delay of CH2 is expected to besignificantly lower than that of CH1, IFS1 (413) may be chosen to belarger than the subsequent IFSs (415, 417, 419) to ensure S1 (412)arrives before the subsequent PPDU. Subsequent IFS durations cansimilarly be configured for expected differences in channel propagationtimes.

Without loss of generality all PPDUs in the example in FIG. 4 areassumed to have a common duration, S. The time-of-arrival (TOA) of thenth PPDU at the kth receiver is denoted by TOAnk. The TOAs of the PPDUsare estimated through receive processing, e.g., by correlating againstpredefined pilot and/or preamble sequences inserted into the PPDUs.

As shown in FIG. 4 , a first receiver (RX1) of the slot detects thefirst PPDU (422) with TOA11+S<T (420-1). The residual time,Δ11=T−(TOA11+S) (423) is large enough for RX1 to attempt to decode S1before the slot expires. However, the subsequent PPDUs (Sn, n=2, 3, 4)are received with TOAn1+S>T, and cannot be processed by RX1.

The second receiver (RX2) shown in FIG. 4 experiences lower propagationdelays across the channels as compared to the first receiver (RX1).Herein, the residual time Δ12=T−(TOA11+S) is large enough for RX2 toattempt to decode the first PPDU, S1 (432) and also (if S1 issuccessfully decoded) to switch to CH2 to detect the second PPDU, S2(434). If the residual time Δ22=T−(TOA22+S) is large enough to processS2 and switch back to the primary channel (CH1) for the subsequent TDMAslot, RX2 decides to decode S2; otherwise S2 is not processed.Subsequent PPDUs (Sn, n=3, 4) are received with TOAn2+S>T, and cannot beprocessed by RX2.

The third receiver (RX3) shown in FIG. 4 experiences even lowerpropagation delays such that TOA13+S<TOA23+S<TOA33+S<T, such that RX3may decode the first three PPDUs. The last PPDU (S4) the slot cannot beprocessed since TOA43+S>T.

Embodiments of the disclosed technology may also be configured forwaveforms with cooperative communication capabilities, for which theremay be multiple transmitters of the same slot, and copies of this slotarrive at a particular receiver with different delays. In order toprevent interference between the PPDUs from multiple transmitters withdifferent propagation delays, each PPDU of a transmitted slot uses adifferent channel, as illustrated in FIG. 5 . As shown therein, S1 (512)is transmitted on CH1 followed by a first interframe spacing IFS1 (513),S2 (514) is transmitted on CH2 followed by a second interframe spacingIFS2 (515), and so on.

In some embodiments, and based on the maximum propagation delay,channels can be reused during the TDMA slot, e.g., CH3=CH1 and CH4=CH2.

According to some embodiments of the disclosed technology, the firstPPDU can be used to transmit a first type of information or data, andthe subsequent PPDU(s) can be used to transmit a second type ofinformation or data. In an example, the first type of data and thesecond type of data are identical. In another example, the second typeof data is different from the first type of data. Subsequent PPDU(s) canbe used for one or more redundant transmissions of data, thetransmission of error tolerant data, and to gain additional usercapacity for short range links.

In some embodiments, subsequent PPDU(s) may be used for redundanttransmissions based on not all links experiencing the maximum expectedpropagation delay. Receivers with smaller propagation delays may receivemultiple copies of the same data within a TDMA slot, and combine thesesignals for improved reliability. This benefit may be particularlyimportant with challenging channels that are subject to signal fading,such as underwater acoustic channels.

In some embodiments, subsequent PPDU(s) can be used for redundanttransmission in a multi-hop Barrage Relay Network (BRN), e.g., asdescribed in the context of FIG. 2 . The Barrage Relay waveform realizessynchronous multi-hop cooperative relay as a part of MAC layerfunctions. The PPDU transmission sequence is associated with adescending propagation delay constraint: the first primary PPDU can bereceived by all links within the maximum propagation delay provisionedby the slot guard time. Each subsequent PPDU can only be received byreceiver(s) subject to lower propagation delays. The successfuldetection & decoding of a PPDU depends on other factors such astransient channel errors. Thus, for BRN, the PPDU sequence automaticallyprovides different multi-hop relay paths consisting of all links, or asubset of short (potentially more reliable) links. This canadvantageously be leveraged to increase the end-to-end path diversityand reliability.

In some embodiments, subsequent PPDU(s) can be used for the transmissionof error tolerant data. In an example, one type of error tolerant datais the Position Location Information (PLI) service that exists in manytactical waveforms. In another example, various Layer 2 control andmanagement functions (including ranging and time tracking between 1-hopneighbors) are error tolerant. In traditional TDMA MAC, such informationconsumes their own time slots, whereas in the described embodiments,they may be advantageously disseminated via the secondary PPDU(s)without additional overhead.

In some embodiments, the subsequent PPDU(s) may be used to supportadditional user capacity for short range links. There are typical MANETnetwork use cases in which the high propagation link(s) only exist in asingle or a few long-haul links, while the majority of the nodes are inclose proximity to their one-hop neighbors. When ranging information isavailable, a cognitive TDMA MAC may intelligently utilize additionalPPDU(s) for majority of short distance traffic, providing long-rangecommunications capability without losing the channel utilizationefficiency.

FIG. 6 shows a flowchart of an example method 600 for high-utilizationlow-latency multi-channel TDMA. The method 600 includes, at operation610, performing, in a first time interval of a time-division multipleaccess (TDMA) slot, a transmission of a first data unit over a firstlogical channel of the plurality of logical channels. In an example, thetransmission may use a constant envelope (CE) waveform, e.g., continuousphase modulation (CPM) or CE orthogonal frequency division multiplexing(CE-OFDM). In another example, the transmission may use OFDM withhigh-order quadrature amplitude modulation (QAM) constellations (e.g.,64-QAM and higher) to achieve a high data rate. In yet other example,the transmission may be more robust by using low-order QAMconstellations (e.g., QPSK, 8-PSK).

The method 600 includes, at operation 620, refraining from transmitting,subsequent to a completion of the transmission of the first data unit,for a second time interval immediately after the first time interval. Inan example, the first data unit may be S1 (412) or S1 (512) in FIGS. 4and 5 , respectively, and the corresponding second time interval may beIFS1 (413) and IFS1 (513), as shown in FIGS. 4 and 5 , respectively.

The method 600 includes, at operation 630, performing (N−1)transmissions in (N−1) time intervals for each data unit of (N−1)subsequent data units in the TDMA slot, such that a transmission of annth data unit of the (N−1) subsequent data units is performed over annth logical channel of the plurality of logical channels. In someembodiments, n and N are positive integers and 2≤n≤N, each of theplurality of logical channels corresponds to a distinct transmissionresource, the wireless medium is characterized by a maximum propagationdelay, and a duration of the TDMA slot is greater than the maximumpropagation delay. Operation 630 is shown, for example, in FIG. 4 forN=4, wherein S2, S3 and S4 are transmitted on CH2, CH3 and CH4,respectively, all within the same TDMA slot (410) of length T.

In some embodiments, the method 600 further includes the operation ofperforming, subsequent to the transmission of the nth data unit, aswitching operation from the nth logical channel to a (n+1)th logicalchannel, wherein 2≤n≤(N−1). As shown in the example in FIG. 4 ,subsequent to transmission of S2 (414), the switching operation (fromCH2 to CH3) is performed in time duration denoted IFS2 (415).

In some embodiments, the distinct transmission resource comprises afrequency, an antenna direction, a polarization, or a code.

In some embodiments, the maximum propagation delay is greater than tensof milliseconds. In an example, the maximum propagation delay of tens ofmilliseconds is due to the wireless medium comprising a line-of-sight(LOS) airborne link over 100s of kilometers. In another example, themaximum propagation delay is due to the wireless medium comprising anunderwater acoustic link with a relatively slow speed of wavepropagation (e.g., ˜1.5 km/sec).

In some embodiments, the second time interval is an intra-burst spacing(e.g., IFS1 denoted 413 and 513 in FIGS. 4 and 5 , respectively).

In some embodiments, a duration of the intra-burst spacing is based onat least the maximum delay spread. In an example, the intra-burstspacing is selected to be large enough to include the switching time,the maximum delay spread, as well as the receiver processing latency ofa data unit, which enables subsequent data units can be processed.

In some embodiments, data in each data unit of the N subsequent dataunits is identical to data in the first data unit. Transmittingredundant data increases the message completion rate (MCR) and/ordecreases the bit error rate (BER) and packet error rate (PER).

In some embodiments, the first data unit comprises high-rate data, andwherein each data unit of the N subsequent data units comprises errortolerant data. In an example, the error tolerant data comprises positionlocation information (PLI) or Layer 2 control and management functions,and the high-rate data comprises video or voice data. In anotherexample, the error tolerant data employs a low-rate error correctingcode (ECC), e.g., low-density parity check (LDPC) code. In yet anotherexample, the high-rate data comprises information with a data rate onthe order of several Megabytes (MBs), whereas the error tolerant datacomprises information with a data rate on the order of a few kilobytes(kB).

In some embodiments, a duration of each of the (N−1) time intervals isidentical to a duration of the first time interval. In an example, thedurations may be identical when the N−1 subsequent data units are usedto retransmit the data transmitted in the first data unit to achieve ahigh MCR or a lower BER/PER via redundant transmissions.

In some embodiments, a duration of each of the N−1 data units may beidentical, but different from a duration of the first time interval. Inan example, this may be the case when the first data unit is used forhigh-rate data and the subsequent data units are used for PLI data.

FIG. 7 shows a flowchart of another example method 700 forhigh-utilization low-latency multi-channel TDMA. The method 700includes, at operation 710, receiving and detecting, at a first time ina time-division multiple access (TDMA) slot, a first data unit over afirst logical channel of the plurality of logical channels.

The method 700 includes, at operation 720, receiving and detecting atleast one of (N−1) subsequent data units at each of (N−1) times in theTDMA slot, such that a reception of an nth data unit of the (N−1)subsequent data units is performed over an nth logical channel of theplurality of logical channels. In some embodiments, n and N are positiveintegers and 2≤n≤N, each of the plurality of logical channelscorresponds to a distinct transmission resource, the wireless medium ischaracterized by a maximum propagation delay, and a duration of the TDMAslot is greater than the maximum propagation delay.

In some embodiments, the method 700 further includes the operation ofperforming, subsequent to the reception of the nth data unit, aswitching operation from the nth logical channel to a (n+1)th logicalchannel, wherein 2≤n≤(N−1).

In some embodiments, detecting the first data unit is based on the firstdata unit comprising a predefined pilot sequence.

In some embodiments, the distinct transmission resource comprises afrequency, an antenna direction, a polarization, or a code.

In some embodiments, a number of the at least one of the (N−1)subsequent data units received and detected is based on a durationbetween the first time and an end of the TDMA slot.

According to embodiments of a disclosed technology, a wirelesscommunication system includes a transmitter, a first receiver coupled tothe transmitter over a wireless medium comprising a plurality of logicalchannels, wherein each of the plurality of logical channels correspondsto a distinct transmission resource, wherein the wireless medium ischaracterized by a maximum propagation delay, and wherein a distancebetween the transmitter and the first receiver corresponds to a firstpropagation delay that is less than the maximum propagation delay, and asecond receiver coupled to the transmitter over the wireless medium,wherein a distance between the transmitter and the second receivercorresponds to a second propagation delay that is less than the firstpropagation delay, wherein the transmitter is configured to perform, ina first time interval of a time-division multiple access (TDMA) slot, atransmission of a first data unit over a first logical channel of theplurality of logical channels to the first receiver and the secondreceiver, refrain from transmitting, subsequent to a completion of thetransmission of the first data unit, for a second time intervalimmediately after the first time interval, and perform (N−1)transmissions in (N−1) time intervals for each data unit of (N−1)subsequent data units in the TDMA slot, such that a transmission of annth data unit of the (N−1) subsequent data units is performed over annth logical channel of the plurality of logical channels, wherein thesecond receiver is configured to receive and decode, within the TDMAslot, at least one of the N subsequent data units, wherein the firstreceiver is unable to receive and decode each of the N subsequent dataunits, wherein a duration of the TDMA slot is greater than the firstpropagation delay, and wherein n and N are positive integers and 2≤n≤N.

In some embodiments, the first time interval greater than or equal to asum of a time required to perform a switching operation between two ofthe plurality of logical channel, a maximum delay spread of the firstlogical channel, and a processing latency of the first data unit.

FIG. 8 is a block diagram representation of a portion of a radio, inaccordance with some embodiments of the presently disclosed technology.A radio 811 can include processor electronics 801 such as amicroprocessor that implements one or more of the techniques presentedin this document. The radio 811 can include transceiver electronics 803to send and/or receive wireless signals over one or more communicationinterfaces such as antenna(s) 809. The radio 811 can include othercommunication interfaces for transmitting and receiving data. Radio 811can include one or more memories 807 configured to store informationsuch as data and/or instructions. In some implementations, the processorelectronics 801 can include at least a portion of the transceiverelectronics 803. In some embodiments, at least some of the disclosedtechniques, modules or functions (including, but not limited to, methods600 and 700) are implemented using the radio 811.

Embodiments of the disclosed technology are directed to overcomingproblems faced by traditional time-slotted MAC schemes operating insystems with large propagation delays. The timeslots in these systemsare normally configured to account for the maximum propagation delay,which results in low utilization or high latencies.

An example technical solution described herein transmits multiple burstson distinct channels within a single TDMA timeslot, with eachtransmitted burst being followed by a time duration in which thetransmitter refrains from transmitting and switches to the channel forthe next burst transmission. Multiple data units are transmitted ondifferent channels within the same timeslot, which advantageouslyenables receivers close to the transmitter to receive and processseveral of the multiple data units, whereas receives further away fromthe transmitter will be able to receive and process at least the firstdata unit that was transmitted.

Some of the embodiments described herein are described in the generalcontext of methods or processes, which may be implemented in oneembodiment by a computer program product, embodied in acomputer-readable medium, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable medium may include removable and non-removable storagedevices including, but not limited to, Read Only Memory (ROM), RandomAccess Memory (RAM), compact discs (CDs), digital versatile discs (DVD),etc. Therefore, the computer-readable media can include a non-transitorystorage media. Generally, program modules may include routines,programs, objects, components, data structures, etc. that performparticular tasks or implement particular abstract data types. Computer-or processor-executable instructions, associated data structures, andprogram modules represent examples of program code for executing stepsof the methods disclosed herein. The particular sequence of suchexecutable instructions or associated data structures representsexamples of corresponding acts for implementing the functions describedin such steps or processes.

Some of the disclosed embodiments can be implemented as devices ormodules using hardware circuits, software, or combinations thereof. Forexample, a hardware circuit implementation can include discrete analogand/or digital components that are, for example, integrated as part of aprinted circuit board. Alternatively, or additionally, the disclosedcomponents or modules can be implemented as an Application SpecificIntegrated Circuit (ASIC) and/or as a Field Programmable Gate Array(FPGA) device. Some implementations may additionally or alternativelyinclude a digital signal processor (DSP) that is a specializedmicroprocessor with an architecture optimized for the operational needsof digital signal processing associated with the disclosedfunctionalities of this application. Similarly, the various componentsor sub-components within each module may be implemented in software,hardware or firmware. The connectivity between the modules and/orcomponents within the modules may be provided using any one of theconnectivity methods and media that is known in the art, including, butnot limited to, communications over the Internet, wired, or wirelessnetworks using the appropriate protocols.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention that is claimed orof what may be claimed, but rather as descriptions of features specificto particular embodiments. Certain features that are described in thisdocument in the context of separate embodiments can also be implementedin combination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or a variation of a sub-combination. Similarly, whileoperations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this disclosure.

What is claimed is:
 1. A method for wireless communication over awireless medium comprising a plurality of logical channels, the methodcomprising: performing, in a first time interval of a time-divisionmultiple access (TDMA) slot, a transmission of a first data unit over afirst logical channel of the plurality of logical channels; refrainingfrom transmitting, subsequent to a completion of the transmission of thefirst data unit, for a second time interval immediately after the firsttime interval; and performing (N−1) transmissions in (N−1) timeintervals for each data unit of (N−1) subsequent data units in the TDMAslot, such that a transmission of an nth data unit of the (N−1)subsequent data units is performed over an nth logical channel of theplurality of logical channels, wherein n and N are positive integers and2≤n≤N, wherein each of the plurality of logical channels corresponds toa distinct transmission resource, wherein the wireless medium ischaracterized by a maximum propagation delay, wherein a duration of theTDMA slot is greater than the maximum propagation delay, wherein thefirst data unit comprises high-rate data, and wherein each data unit ofthe (N−1) subsequent data units comprises error tolerant data.
 2. Themethod of claim 1, further comprising: performing, subsequent to thetransmission of the nth data unit, a switching operation from the nthlogical channel to a (n+1)th logical channel, wherein 2≤n≤(N−1).
 3. Themethod of claim 1, wherein the distinct transmission resource comprisesa frequency, an antenna direction, a polarization, or a code.
 4. Themethod of claim 1, wherein the maximum propagation delay is greater thantens of milliseconds.
 5. The method of claim 1, wherein the second timeinterval is an intra-burst spacing.
 6. The method of claim 5, wherein aduration of the intra-burst spacing is based on at least a maximum delayspread of the wireless medium.
 7. The method of claim 1, wherein thewireless medium comprises a line-of-sight (LOS) airborne link or anunderwater acoustic link.
 8. The method of claim 1, wherein the errortolerant data comprises position location information (PLI) or Layer 2control and management functions.
 9. The method of claim 1, wherein aduration of each of the (N−1) time intervals is identical to a durationof the first time interval.
 10. A wireless communication system,comprising: a transmitter; a first receiver coupled to the transmitterover a wireless medium comprising a plurality of logical channels,wherein each of the plurality of logical channels corresponds to adistinct transmission resource, wherein the wireless medium ischaracterized by a maximum propagation delay, and wherein a distancebetween the transmitter and the first receiver corresponds to a firstpropagation delay that is less than the maximum propagation delay; and asecond receiver coupled to the transmitter over the wireless medium,wherein a distance between the transmitter and the second receivercorresponds to a second propagation delay that is less than the firstpropagation delay, wherein the transmitter is configured to: perform, ina first time interval of a time-division multiple access (TDMA) slot, atransmission of a first data unit over a first logical channel of theplurality of logical channels to the first receiver and the secondreceiver, refrain from transmitting, subsequent to a completion of thetransmission of the first data unit, for a second time intervalimmediately after the first time interval, and perform (N−1)transmissions in (N−1) time intervals for each data unit of (N−1)subsequent data units in the TDMA slot, such that a transmission of annth data unit of the (N−1) subsequent data units is performed over annth logical channel of the plurality of logical channels, wherein thesecond receiver is configured to receive and decode, within the TDMAslot, at least one of N subsequent data units, wherein the firstreceiver is unable to receive and decode each of the N subsequent dataunits, wherein a duration of the TDMA slot is greater than the firstpropagation delay, wherein n and N are positive integers and 2≤n≤N, andwherein the second time interval is greater than or equal to a sum of atime required to perform a switching operation between two of theplurality of logical channel, a maximum delay spread of the firstlogical channel, and a processing latency of the first data unit. 11.The system of claim 10, wherein size of the first data unit and a sizeof each of the (N−1) subsequent data units is identical.
 12. The systemof claim 10, wherein the distinct transmission resource comprises afrequency, an antenna direction, a polarization, or a code.
 13. Thesystem of claim 10, wherein the maximum propagation delay is greaterthan tens of milliseconds.
 14. The system of claim 10, wherein thesecond time interval is an intra-burst spacing.
 15. The system of claim14, wherein a duration of the intra-burst spacing is based on at least amaximum delay spread of the wireless medium.
 16. The system of claim 10,wherein the wireless medium comprises a line-of-sight (LOS) airbornelink or an underwater acoustic link.
 17. The system of claim 10, whereinthe first data unit comprises high-rate data, and wherein each data unitof the (N−1) subsequent data units comprises error tolerant data. 18.The system of claim 17, wherein the error tolerant data comprisesposition location information (PLI) or Layer 2 control and managementfunctions.